More particularly, the invention relates to a hybrid helicopter of the vertical takeoff and landing (VTOL) advanced concept type of aircraft. Given the specificity of the invention, its technical field is restricted to vertical takeoff and landing rotorcraft having a main lift rotor acting as a rotary wing, and the technical field does not extend to airplanes, for example, regardless of whether or not they permit vertical takeoff and landing.
Indeed, the aerodynamic constraints and problems to be solved, e.g. on rotary wing aircraft, differ significantly from the constraints and problems encountered with fixed wing aircraft, which explains why they are not included in the technical field of the invention.
This advanced concept of a hybrid helicopter combines, at reasonable cost, the efficiency of a conventional helicopter in vertical flight with high speed travel performance made possible by the use of pusher propellers and the installation of modern turbine engines.
In order to understand the object of the invention, it is appropriate to summarize the main flying machines that correspond to airplanes and to rotorcraft.
The term “rotorcraft” is used to designate any aircraft for which lift is provided in full or in part by one or more substantially vertical axis propellers of large diameter, referred to as rotors or as rotary wings.
Within the category of rotorcraft, several distinct types can be distinguished.
Firstly, there is the helicopter in which at least one main rotor driven by an appropriate engine provides both lift and propulsion. A helicopter is capable of hovering, i.e. remaining at a stationary point in three dimensions, it can take off and land vertically, and it can move in any direction (forwards, backwards, sideways, up, down). The great majority of rotorcraft that are produced in the world are helicopters.
Then the autogyro is a rotorcraft in which the rotor does not receive power, but provides lift by auto-rotating under the effect of the forward speed of the aircraft. Propulsion is provided by a turbine engine or by a propeller having a substantially horizontal axis in forward flight that is driven by a conventional engine. This formula is incapable of vertical flight, except when the rotor is initially launched by an auxiliary device enabling the rotor to be rotated faster than normal: an autogyro therefore cannot perform hovering flight, but is capable of moving up or down on paths of very steep slope. It could be said to be an airplane with a wide range of speeds that does not suffer from stalling and that can use short runways.
A gyrodyne is a rotorcraft that is intermediate between a helicopter and an autogyro, with a rotor that provides only lift. The rotor is normally driven by an engine installation during stages of takeoff, hovering or vertical flight, or landing, like a helicopter. A gyrodyne also has an additional propulsion system that is essentially different from the rotor assembly. In forward flight, the rotor continues to provide lift, but only in auto-rotation, i.e. without power being transmitted to said rotor. The Jet gyrodyne from the manufacturer Fairey is an embodiment of this concept.
Various other novel formulae have been investigated to a greater or lesser extent, some of which have given rise to practical embodiments.
In this respect, mention can be made of the compound that takes off and lands like a helicopter, and cruises like an autogyro: its rotor also rotates because of the forward speed of the aircraft and provides part of its lift, while the remainder is provided by an auxiliary wing. A puller propeller having a substantially horizontal axis delivers the force needed to move in translation. As an example, mention can be made of the experimental compound SO 1310 Farfadet having its rotor propelled by jet propulsion in a takeoff or landing configuration and also rotating in a cruising configuration, propulsion then being provided by a propeller. That aircraft has two separate turbines for actuating the rotor and the propeller.
Similarly, document U.S. Pat. No. 6,513,752 discloses an aircraft comprising:                a fuselage and a wing;        two variable-pitch propellers;        a rotor with weights “at its ends” (sic);        a power source driving the two propellers and the rotor;        control means for adjusting the pitch of the propeller such that:                    in forward flight, the thrust from the propellers causes the aircraft to move forward; and            in hovering flight, the anti-torque function is performed by one propeller providing thrust towards the front of the rotorcraft and the other towards the rear of the rotorcraft, and the rotor is driven by the power source; and                        the power source comprises an engine and a clutch, which by disconnecting the rotor from the engine, enables the rotor to turn faster than an outlet from said engine, because of the above-mentioned weights.        
Additionally, it is stated that the clutch makes autogyro mode in forward flight possible. Consequently, the aircraft according to document U.S. Pat. No. 6,513,752 is of the compound type.
In addition, the power transmission gearbox disposed between the power source and the propellers enables said propellers to operate at a plurality of speeds of rotation relative to the speed of an outlet from said power source.
Another particular formula for a rotorcraft is the convertible. This term covers all rotorcraft that change configuration in flight: takeoff and landing in helicopter configuration, cruising flight in airplane configuration, e.g. with two rotors that are tilted through about 90° in order to act as propellers. The tilting rotor formula has given rise to the Bell Boeing V22 Osprey aircraft, for example.
Of these various rotorcraft formulae, the helicopter is the simplest, such that is the most widespread in spite of the fact that the maximum forward speed of a helicopter is about 300 kilometers per hour (km/h), which is slow and less than the speed that can be envisaged with formulae of the compound or convertible types, which are technically more complex and more expensive.
Another innovative formula is known that is referred to as a “hybrid helicopter”, for convenience.
The hybrid helicopter comprises a fuselage with a cockpit at its front, a main rotor for driving blades in rotation by using at least one turbine engine, and advantageously by using two turbine engines placed on the top of the fuselage one either side of the longitudinal plane of symmetry of the aircraft.
The hybrid helicopter also has a set of wings made up of straight (i.e. non-swept) wings disposed on either side of the fuselage. Two pusher propellers are placed on either side of the fuselage, on the wings.
Furthermore, the hybrid helicopter has a tail boom with stabilizer and control surfaces at its end, and in particular: for controlling pitch, a horizontal stabilizer surface with two pitch control surfaces that are movable relative to the front portion; and, for steering, two appropriate fin surfaces, e.g. substantially vertical surfaces, on respective sides of the horizontal stabilizer.
Specifically, the horizontal stabilizer and the vertical stabilizers form an upside-down U-shape relative to the fuselage.
Furthermore, the hybrid helicopter is fitted with an integrated drive system that, in addition to comprising the two turbine engines, the rotors, and the two propellers, also comprises a mechanical interconnection system between those elements.
In that configuration, the hybrid helicopter is remarkable in that the speeds of rotation of the turbine engine outlets, of the propellers, of the rotor, and of the mechanical interconnection system are mutually proportional, with the proportionality ratio being constant regardless of the flying configuration of the hybrid helicopter under normal conditions of operation of the integrated drive system.
Consequently, and advantageously, the rotor is continuously driven in rotation by the turbine engine(s), and always develops lift regardless of the configuration of the hybrid helicopter, both in forward flight and when hovering. The hybrid helicopter is thus neither an autogyro, nor a gyrodyne, nor a compound, but is a novel type of rotorcraft.
More precisely, the rotor serves to provide all of the lift of the hybrid helicopter during stages of takeoff, landing, and hovering, and some of its lift in cruising flight, the wing then contributing some of the lift of said hybrid helicopter.
Thus, the rotor provides the major fraction of the lift of the hybrid helicopter in cruising flight, and possibly also makes a small contribution to propulsion or traction forces, while minimizing drag.
The anti-torque and steering control functions are performed by making use of differential thrust exerted by the propellers. In vertical flight, the propeller on the left of the fuselage exerts thrust towards the rear of the hybrid helicopter while the propeller on the right produces thrust towards the front, assuming that the rotor turns clockwise.
The hybrid helicopter makes it possible not only to perform missions that take a long time in vertical flight or in cruising flight at high speed, but also makes it possible to have long range, while still being capable of hovering and taking off vertically.
Nevertheless, the propellers are close to fuselage, which gives rise to noise nuisance for the passengers of a hybrid helicopter.
In addition, the tail boom can give rise to vibration known to the person skilled in the art as “tail shake”. It should be observed that this “tail shake” phenomenon applies to rotorcraft only.
In practice, the main rotor behaves like an aerodynamic exciter. Thus, its wake is turbulent. Turbulence corresponds to variations in pressure, speed, and angle of incidence of the slipstream, which variations are distributed over a rather wide range of relatively high frequencies.
The wake behind the main rotor of the hybrid helicopter is pulsed at the fundamental frequency equal to the product b×Ω, where b is the number of blades of the main rotor and Ω is the speed of rotation of the rotor.
Nevertheless, frequencies that are harmonics of b×Ω may possibly also appear.
Under such conditions, the tail surfaces are subjected simultaneously to such aerodynamic excitation, thereby leading directly to exciting resonance modes of the helicopter structure.
It should also be observed that the exhaust gas leaving the turbine engines heats the rear structural assemblies comprising the tail boom and the tail surfaces, which can give rise to problems in finding materials to constitute these rear structural assemblies.
The vibration that results from “tail shake” can also be a source of discomfort for passengers. The “tail shake” phenomenon is then combated with the help of active devices, e.g. devices that act on the tail surfaces or indeed on the tail boom.